If cocaine (and it's metabolites) from a single use are no longer in your urine, would they also be out of breastmilk?

Yes: Cocaine is very short-acting, and the active drug is in the bloodstream, and therefore can get into breast milk, for perhaps 8 hours. Within 24 hours there would be no chance that cocaine itself would enter breastmilk. The metabolites are what are measured in urine, and these are not metabolically active anyway. However, if you are using cocaine and have a child, you really need to seek help!!!